Phoresy in Lice

Phoretic relationships between lice and Hippoboscid flies

Despite belonging to a group of winged insects, lice have secondarily
lost their wings in favour of a more passive form of transmission from
host to host. Most lice are transmitted by physical contact with mates
during breeding to ensure that the lice remaining on a suitable host
species. Thus its been said that lice are passed from one host generation
to the next like genetic heirlooms. However, a number of alternative
possible mechanisms for transmission exist, and perhaps the most famous
of these involves the louse-fly (Hippoboscidae).
These flies are frequently found around many birds and there are a number
of reports of lice found attached to them, particularly after the death
of the host. This behavior by the lice is known as phoresy and is in
effect a risky process of redistribution in which the lice get to travel
freely on the flies without feeding on them. There are many records
of lice found attached to Hippoboscid flies, although in almost all
instances the lice concerned are members of the suborder Ischnocera.
This type of transmission allows the louse to escape a dead or dying
host in an attempt to find a more favourable host environment. Nevertheless,
there is a high element of risk in the process as the flies are not
specific to particular host species and are likely to carry the lice
to an unsuitable host on which they will be unable to establish. This
type of behaviour is important when studying the cospeciation of lice and their hosts as it could
provide an explanation for some of the instances in which lice have
switched from one host species to another during their coevolutionary
history.